Focused charged particle beam systems, such as ion and electron beam systems, are used to image, analyze, and modify samples on a microscopic or nanoscopic scale. An electron beam, for example, can be used in a scanning electron microscope (SEM) to form images of a sample by the detection of secondary electrons and for elemental analysis by measuring an x-ray spectrum. A focused ion beam (FIB) system, for example, can be used in a variety of applications in nanotechnology and integrated circuit manufacturing to create and alter microscopic and nanoscopic structures. FIB systems can be used, for example, to image, mill, deposit, and analyze with great precision. Milling or micromachining involves the removal of bulk material at or near the surface of a sample. Milling can be performed without an etch-assisting gas, in a process called sputtering, or using an etch-assisting gas, in a process referred to as chemically-assisted ion beam etching. U.S. Pat. No. 5,188,705, which is assigned to the assignee of the present invention, describes a chemically-assisted ion beam etching process. In chemically-assisted ion beam etching, an etch-enhancing gas reacts in the presence of the ion beam to combine with the surface material to form volatile compounds. In FIB deposition, a precursor gas, such as an organometallic compound, decomposes in the presence of the ion beam to deposit material onto the target surface.
The charged particles can be produced from a variety of sources. A “bright” source is desirable because it can produce more charged particles into a smaller spot. The “brightness” of a charged particle beam source is related to the number of charged particles emitted per area and the solid angle into which the particles are emitted. The area from which the particles appear to be emitted is referred to as the “virtual source.” High brightness sources typically have small virtual source sizes and can be focused onto smaller spots, which provides for higher resolution processing.
A liquid metal ion source (LMIS) is very bright and can provide high resolution processing, but is limited to a low beam current at high resolution. A typical system using a gallium LMIS can provide five to seven nanometers of lateral resolution. Such systems are widely used in the characterization and treatment of materials on microscopic to nanoscopic scales. A gallium LMIS comprises a pointed needle coated with a layer of gallium. An electric field is applied to the liquid gallium to extract ions from the source. To produce a very narrow beam for high resolution processing, the current in a beam from an LMIS must be kept relatively low, which means low etch rates and longer processing times. As the beam current is increased beyond a certain point, the resolution rapidly degrades.
Plasma ion sources ionize gas in a plasma chamber and extract ions to form a beam that is focused on a work piece. Plasma ion sources have larger virtual source sizes than LMISs and are much less bright. An ion beam from a plasma source, therefore, cannot be focused to as small of a spot as the beam from an LMIS, although a plasma source can produce significantly more current. Plasma sources, such as a duoplasmatron source described by Coath and Long, “A High-Brightness Duoplasmatron Ion Source Microprobe Secondary Ion Mass Spectroscopy,” Rev. Sci. Instruments 66(2), p. 1018 (1995), have been used as ion sources for ion beam systems, particularly for applications in mass spectroscopy and ion implantation. Inductively coupled plasma (ICP) sources have been used more recently with a focusing column to form a focused beam of charged particles, i.e., ions or electrons.
An inductively coupled plasma source is capable of providing charged particles within a narrow energy range, which allows the particles to be focused to a smaller spot than ions from a duoplasmatron source. ICP sources, such as the one described in U.S. Pat. No. 7,241,361, which is assigned to the assignee of the present invention, include a radio frequency (RF) antenna typically wrapped around a ceramic plasma chamber. The RF antenna provides energy to maintain the gas in an ionized state within the chamber. Because the virtual source size of a plasma source is much larger than the virtual source size of an LMIS, the plasma source is much less bright.
Electron beams are used in a scanning electron microscope (SEM) to form images of a work piece. The electrons are typically provided by a tungsten or lanthanum hexaboride thermionic emitter or a field emission gun, such as a Schottky emitter or a cold cathode emitter. Such emitters provide a small virtual source and can be focused to a very small spot. A typical electron emitter used in an SEM may have a reduced brightness of between about 2×107 A/m2·sr·V and 8.2×107 A/m2·sr·V. Electrons can be focused to a smaller spot than ions and cause less damage to the sample.
An electron beam focusing column is significantly different from an ion beam focusing column. For example, electron beam columns typically uses magnetic focusing lenses because of their lower aberration, whereas ion beam columns typically use electrostatic focusing lenses because an excessively large current would be required to focus the heavy ions using a magnetic lens.
An SEM can be used not only to form an image of a work piece, but also to analyze the processed area for chemical or elemental composition. Energy dispersive spectroscopy (EDS) systems are commonly found on SEMs and are useful in performing localized chemical analysis of a sample. EDS systems utilize the x-ray spectrum emitted by a material impacted by high-energy electrons from the focused beam of electrons of the SEM imaging operation. When an electron impacts the sample, the electron loses energy by a variety of mechanisms. One energy loss mechanism includes transferring the electron energy to an inner shell electron, which can be ejected from the atom as a result. An outer shell electron will then fall into the inner shell, and a characteristic x-ray may be emitted. The energy of the characteristic x-ray is determined by the difference in energies between the inner shell and the outer shell. Because the energies of the shells are characteristic of the element, the energy of the x-ray is also characteristic of the material from which it is emitted. When the number of x-rays at different energies is plotted on a graph, one obtains a characteristic spectrum, such as the spectrum of pyrite shown in FIG. 1. The peaks are named for the corresponding original and final shell of the electron that originated the x-ray. FIG. 1 shows the sulfur Kα peak, the iron Kα peak and the iron Kβ peaks.
It is often useful to prepare a sample using a FIB and then use an SEM to image or to perform an EDS analysis. If two separate instruments are used, the user is faced with a lengthy process of removing the sample from the FIB system and then setting up the sample in the EDS capable system which can take upwards of 30 minutes or more. This makes for an unacceptably long time for many applications to be able to analyze a prepared sample.
A known solution to reduce this overhead is a dual-beam system consisting of a FIB column and SEM column on the same platform. Each column is optimized for the type of particle beam that it produces. A dual column system is costly because of the two separate focusing columns. The SEM of a dual beam system, such as the Quanta 3D system from FEI Company, the assignee of the present invention, typically has an imaging resolution of better than five nanometers.
While it is known that a plasma chamber can be used as a source of electrons as well as ions, plasma sources are not typically used for an SEM because the large virtual source size makes for poor resolution compared to other sources. Moreover, when electrons are extracted from a plasma source, negative ions are also extracted. Lastly, because the difference in the configuration of the optical columns for focusing electron and focusing ions, using the same column for both ions and electrons would result in less than optimum resolution for ions, electrons, or both.